Heretofore, thermoplastic materials such as polystyrene or polyethylene foam materials have been trimmed by heat-cutting with a trim tool. This creates a ridge commonly termed flash on the formed side of the material, which must be removed in a separate trimming operation. It also leaves a residue on the trim tool, which is rarely heated to a temperature high enough to vaporize the material. This residue must frequently be removed in order that the trim tool may perform an effective trimming operation.
Examples of prior art apparatus and processes are in the patents to Huppenthal et al U.S. Pat. Nos. 3,707,102, Haase 3,684,633, Seto 3,475,526, Byrne 3,572,424, Langdon et al 3,362,043 and Scalora 3,240,851.
In accordance with the principles of the present invention, trimming of the thermoplastic material may be attained without exerting pressure on the material and the flash or ridge formerly formed by the trim tool is avoided by pressurizing the material along the edge to be trimmed by a trim guide or mold in the form of two pieces of metal of high heat-conductivity, which usually are in the form of cooperating mold parts between which the material is placed when molded to form. These two mold parts or trim guides are made from metal of high heat-conductivity and may be moved relative to each other to pressurize the material along the area to be trimmed.
The plastic material may be a polystyrene or polyethylene foam of various densities, and may have a lamina of high-density polystyrene film on one side thereof which may be an oriented thermoplastic film forming an outer lamina of high-density.
A trim tool heated to a temperature high enough to melt the thermoplastic material, which may even be high enough to vaporize the material, is then used to trim the material along the trim guide and provide a cleanly trimmed edge without touching the thermoplastic material. The trimmed edge may be an internal or external edge and when so formed, has no flash or heat ridge.
The trim guide is preferably made from aluminum, but may be made from other metals having a high heat-conductivity, and holds and pressurizes the material along the area to be trimmed and absorbs the heat of trimming so the heat ridge or flash formerly present in heat-trimming foam materials has no place to form. The trim guide may be the parts of a mold or die, and may be formed to accommodate a heated trim tool to trim the external periphery of an article to be formed or may be formed to accommodate a heated trim tool to place slots or apertures of various forms in foam material as the material is pressurized along the portion to be slotted or apertured as the heated trim tool performs the slotting or aperturing operation.
The advantages of the present invention are that the deficiencies of heat-trimming thermoplastic foam materials, such as polystyrene or polyethylene foam are obviated by pressurizing the thermoplastic material to be trimmed along its edge or perimeter to be trimmed by the use of two pieces of metal of high heat-conductivity, and trimming the material along the two pieces of metal by a heated trim tool, which may be hot enough to vaporize the material along the pieces of metal, in which the pieces of metal form a heat barrier at which the melting of the thermoplastic material stops.
Another advantage of the invention is that the heat ridge heretofore formed by trimming thermoplastic materials and particularly foam materials with a hot trim tool is obviated by the use of a trim tool hot enough to melt the material and moving this tool relative to surfaces of high heat-conductivity, pressurizing the edge of the material trimmed, and maintained cold enough to provide a heat barrier, absorbing the heat of melting.
A still further advantage is that by confining the foam material between two pieces of metal of high heat-conductivity and exerting pressure on the material along the edge of perimeter to be trimmed, a definitive point is created at which melting by the heated trim tool stops.
Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.